Changes in proximate composition and estimates of energetic costs during upstream migration and spawning in Atlantic salmon Salmo salar

1. There were marked changes in the relative lipid content of somatic and visceral tissues during the course of upstream migration and spawning. The lipid content per 100 g wet tissue mass was high in maturing salmon in July (soma, pooled sexes: 12.1 g; viscera: 11.4 g in males and 7.3 g in females) and decreased to a minimum after spawning in November (soma: 2.1 g; viscera: 1.6 g). Percentages of protein in the soma and viscera were relatively stable during the study period, ranging from 21.9 g (soma) and 16.3 g (viscera) 100 g(-1) tissue in maturing fish in September-October, to 17.1 g (soma) and 16.1 g(viscera) in spent salmon. The carbohydrate content was relatively small, ranging between 0.27 and 2.4 g 100 g(-1) wet tissue mass. 2. Application of energetic coefficients suggested that the somatic energy content decreased from 814 kJ 100 g(-1) wet tissue mass in maturing salmon (pooled sexes) caught in coastal waters in July, to 370 kJ in spent fish in the river in November. Visceral energy density decreased in the same period, from 706 kJ in males and 570 kJ in females to 340 kJ and 360 kJ, respectively. 3. Mean gonadosomatic index (GSI) values, in terms of energy of males, in July and just prior to spawning were 0.4 and 3.2, respectively. The corresponding figures for females were 2.8 and 28.6. 4. Gonadal energy contents per unit tissue mass were estimated to be higher in females than males, except for spent fish. Prior to spawning energy content was 845 kJ 100 g(-1) gonad wet mass in females and 375 kJ in males. After spawning the gonadal energy contents were 280 kJ and 385 kJ 100 g(-1) wet mass, respectively. 5. The relative energy, lipid and protein contents of somatic, visceral and gonadal tissues were highly correlated with percentage tissue water. 6. Total energy loss due to migration and spawning was between 60% and 70% of the body reserves prior to upstream migration. It was similar for males and females, but higher in large than small salmon. Estimated energy investment due to reproduction correlated significantly with estimated survival rates in 12 Norwegian populations of Atlantic salmon.